When Stalin demanded in 1944 that all Soviet Union republics be admitted to the UN, he revealed a conception of sovereignty that diverged from the usual perception of Soviet diplomacy as exceedingly centralised. Soviet theories and practices of sovereignty consisted indeed in a mix of contradictory elements, illustrating the communist criticism of bourgeois international law, but also a willingness to re-use parts of it and tailor them to new political needs. This article focuses on this elastic approach to sovereignty, its legal expression and diplomatic rationale. Particular attention is paid to the sovereignty of Union republics, central to Soviet legal rhetoric, that led them to be active in the international arena in the 1920s and after 1944, and develop state institutions that would smooth up the transition to independence after 1991.